1. Technical Field
This disclosure generally relates to semiconductor devices and methods for manufacturing the semiconductor devices. More specifically, the disclosure relates to a semiconductor device having a power MOSFET and a method for manufacturing such a semiconductor device. Here, the power MOSFET includes a trench formed on a surface of a semiconductor layer forming a drain, a gate electrode formed in the trench via a gate insulation film, a channel diffusion layer formed at a surface side of the semiconductor layer, shallower than the trench and neighboring the trench, and a source diffusion layer formed at a surface side of the channel diffusion layer and neighboring the trench.
2. Description of the Related Art
Conventionally, a trench gate type power MOSFET (metal oxide semiconductor field effect transistor) is used as a MOSFET for which high speed switching is required such as a MOSFET used for a DC/DC converter or the like. In the power MOSFET, in order to improve the switching properties, it is required to make an on-resistance (resistance between a source electrode and a drain electrode of FET in a state where FET is in on-state) and feedback capacitance (capacitance between a gate and a drain) small.
However, the on-resistance and the feedback capacitance are in conflict with each other. In other words, while it is necessary to make an insulation film of a trench bottom part projecting to a drain electrode thick in order to reduce the feedback capacitance, the on-resistance becomes high if the gate insulation film of a trench side surface is made thick.
In order to solve such a problem, for example, a method whereby the insulation film of the trench bottom part is made thicker than the thickness of the gate insulation film is suggested in Japanese Patent No. 3699907.
In the method whereby only the insulation film of the trench bottom part is made thick, a silicon oxide film is used as the insulation film and a silicon nitride film as an oxidation resistance film is formed on the trench side surface at the time of thermal oxidation. As a result of this, oxidation of the trench side surface is prevented and only the trench bottom part is oxidized.
In addition, a method is suggested wherein neutral particle such as argon are implanted with high concentration so that silicon crystal of the trench bottom part is made amorphous and the trench bottom part is promoted.
Furthermore, as another method for solving the above-discussed problem, for example, Japanese Laid-Open Patent Application Publication No. 10-173175 suggests a method wherein an impurity ion concentration at the trench bottom side of gate poly-silicon embedded in the trench is decreased or poly-silicon not containing impurity ions is embedded in the trench bottom part so that a capacitor is formed inside the gate poly-silicon and the feedback capacitance is reduced.